Tests that cookie blocking is applied to ping loads.
   

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Frame: '<!--frame1-->'
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Should receive first-party cookie.
Received cookie named 'firstPartyCookie'.
Client-side document.cookie: firstPartyCookie=value

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Frame: '<!--frame2-->'
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Should receive no cookies.
Did not receive cookie named 'firstPartyCookie'.
Client-side document.cookie:

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Frame: '<!--frame3-->'
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Ping received.
HTTP_HOST: localhost:8000
REQUEST_URI: /contentextensions/resources/save-ping.php?test=ping-to-prevalent-resource
No cookies in ping.